The iPad needs 10w of charge to charge correctly this is not achievable from all but the very latest USB ports
and that is why the iPad has a 2A powerpack .
The normal iPhone charger is 5w (1Amp) and therefore charges an iPhone correctly as does the USB port
With this in mind your iPad would appear to behave strangely If you have carried out normal troubleshooting
procedures inc a restore maybe a Genius Bar visit is called for
"The fastest way to charge your iPad is with the included 10W USB Power Adapter.
iPad will also charge, although more slowly, when attached to a computer with a high-power USB port (many recent Mac computers) or with an iPhone Power Adapter.
When attached to a computer via a standard USB port (most PCs or older Mac computers) iPad will charge, but only when it's in sleep mode.
Make sure your computer is on while charging iPad via USB. If iPad is connected to a computer that’s turned off or is in sleep or standby mode, the iPad battery will continue to drain."
Ipad Battery About Best Practice